Minimum Ductility Research Articles

Fibre-reinforced concrete beam assemblages subject to column loss

Application of steel-fibre-reinforced concrete (SFRC) to partially replace conventional steel reinforcement, particularly the stirrups in beams, has found increasing use in practice. However, the collapse response of SFRC beams (with no stirrups) subject to column loss scenario has not been investigated. Accordingly, four 2/5th scale beam sub-assemblages with SFRC and no stirrups are constructed and tested under a column loss scenario and the potential for steel fibres to replace the stirrups is investigated. The experimental results show that the use of structural steel fibres as a replacement for stirrups has a minor influence on the membrane behaviour of beams as well as the robustness of SFRC frames subjected to column loss. In addition, detailed finite-element models of the assemblages are developed and validated against the experimental data and the validated models are used to investigate the influence of steel bar ductility on the ability of beams to develop tensile membrane action. It is concluded that the minimum ductility requirements adopted by the Australian standard for normal ductility reinforcing bars (minimum uniform elongation of 0·05) is not sufficient to allow full development of catenary action in reinforced concrete/SFRC beams.

Fibre-reinforced concrete beam assemblages subject to column loss

Application of steel-fibre-reinforced concrete (SFRC) to partially replace conventional steel reinforcement, particularly the stirrups in beams, has found increasing use in practice. However, the collapse response of SFRC beams (with no stirrups) subject to column loss scenario has not been investigated. Accordingly, four 2/5th scale beam sub-assemblages with SFRC and no stirrups are constructed and tested under a column loss scenario and the potential for steel fibres to replace the stirrups is investigated. The experimental results show that the use of structural steel fibres as a replacement for stirrups has a minor influence on the membrane behaviour of beams as well as the robustness of SFRC frames subjected to column loss. In addition, detailed finite-element models of the assemblages are developed and validated against the experimental data and the validated models are used to investigate the influence of steel bar ductility on the ability of beams to develop tensile membrane action. It is concluded that the minimum ductility requirements adopted by the Australian standard for normal ductility reinforcing bars (minimum uniform elongation of 0·05) is not sufficient to allow full development of catenary action in reinforced concrete/SFRC beams.

Mechanism of hot ductility loss in C–Mn steels based on nonequilibrium grain boundary segregation of impurities

Abstract

Heat Treatment of Heat-Resistant Ferrous Cast Alloys

Heat-resistant ferrous cast alloys are generally divided into four groups: ferritic cast iron, austenitic ductile iron, ferritic stainless steel, and austenitic stainless steel. There is an active debate about whether or not heat treatment should be imposed on these ferrous castings prior to placing them in service. In this investigation, a series of experiments were conducted to evaluate the effect of heat treatment on the microstructure, tensile properties, hardness, linear growth, and thermal fatigue life for alloys of each of the four groups. In general, heat treatment appeared to cause similar microstructural changes in these alloys such as the formation of precipitated particles adjacent to the cell boundary regions, but it also affected each material’s properties in a different fashion. In spite of a slight increase in tensile elongation at room temperature, annealing had no impact on the ductility minima at medium temperatures for ferritic cast iron. Heat treatment did not appreciably alter the tensile properties of austenitic ductile iron. The Brinell hardness of ferritic stainless steel samples was reduced, but the hardness was increased for austenitic stainless steel samples after heat treatment.

Hot Ductility Behavior of Boron Containing Microalloyed Steels with Varying Manganese Contents

The hot ductility is measured for six different steel grades with different microalloying elements and with varying manganese contents using the hot tensile test machine with melting/solidification unit at the Department of Ferrous Metallurgy RWTH Aachen University. To identify the influence of manganese on hot ductility, tests are performed with varying the manganese content from 0.7 to 18.2 wt pct, a high manganese steel. Additionally, the effect of different cooling and strain rates is analyzed by changing the particular rate for selected samples in the minima. To investigate and detect the cause of cracking during testing, the fracture surfaces in the ductility minima are considered with scanning electron microscope–energy dispersive X-ray spectroscopy. Thermodynamic modeling is conducted on basis of the commercial software ThermoCalc©. A sharp decrease of the hot ductility is recognizable at 1398 K (1125 °C), at only 0.7 wt pct manganese because of the low manganese to sulfur ratio. The grades with a Mn content up to 1.9 wt pct show a good ductility with minimal ductility loss. In comparison, the steel grade with 18.2 wt pct has a poor hot ductility. Because of the formation of complex precipitates, where several alloying elements are involved, the influence of boron on hot ductility is not fully clarified. By increasing the cooling rate, the reduction of area values are shifted to smaller values. For high test temperatures, these measured values are decreased for lower strain rates. Thereby, an early drop of the ductility is noticeable for the high temperatures around 1373 K (1100 °C).

Processing of interpenetrating Mg–TiNi composites by spark plasma sintering

Mg/Ti49.4Ni50.6 interpenetrating composites have been produced via spark plasma sintering technique. Interpenetrating matrix and the reinforcement networks were formed simply by adjustment of the particle sizes of the starting pure Mg and TiNi alloy powders. Microstructural characterization revealed a good interfacial bonding, i.e. free from porosity, oxide and intermetallic compounds, between Mg matrix and the reinforcement. It has been found that, the microstructure of TiNi shape memory alloy reinforcement material is highly sensitive to processing conditions especially to sintering time. Different sintering times resulted in different phase formations and accordingly different phase transformation temperatures due to the overlap of sintering temperature with aging temperature. Room temperature yield and compressive strength of the composite samples were in the range of 53–113 MPa and 226–315 MPa respectively, depending on the reinforcement content (10–30 vol%) and the texture formation during processing. Ductility of the composites on the other hand was in the range of 9.6–20%. 30% TiNi addition resulted in minimum ductility, whereas composite samples with 20% TiNi reinforcement exhibited the maximum ductility values. Mg/TiNi composites displayed extraordinary mechanical properties at high temperatures. Yield strength and elastic modulus of the composites increased with increasing temperature up to 150 °C due to the increase of transformation stress and the elastic modulus of TiNi shape memory reinforcements with increasing temperature. Beyond 150 °C both properties decreased with further temperature increase. TiNi alloys seem to be promising candidates for reinforcement of pure Mg and Mg alloys to be used in automotive powertrain applications. Composites produced with 20% and 30% TiNi content meet the high strength and elastic modulus requirements at the service conditions of 150–200 °C and the stress levels of 50–70 MPa.

Hot ductility behavior of near-alpha titanium alloy IMI834

Abstract The hot ductility of rolled IMI834 titanium alloy has been studied by conducting tensile tests with a strain rate of 0.1 s−1 and temperature range of 750–1 100 °C to obtain the optimum hot working conditions. The alloy showed minimum hot ductility in the lower alpha–beta region in the temperature range 750–950 °C. Further microstructural characterizations showed improvement in hot ductility by increasing temperature, which was attributed to reduction of volume fraction of high strength alpha phase. The best hot ductility was observed at 1 000 °C, i. e. in the upper alpha–beta region. The better hot ductility at higher temperature could be related to the increase in the volume fraction of beta phase and the occurrence of dynamic restoration phenomena. The second decline in hot ductility appeared at higher temperatures in the beta region and was attributed to the high stacking fault energy and self-diffusion of beta phase leading to limitation of dynamic recrystallization.

Dynamic strain ageing, deformation, and fracture behavior of modified 9Cr–1Mo steel

Tensile behavior of modified 9Cr–1Mo steel was studied over the temperature range from room temperature (RT) to 600 °C at three strain rates 10−5, 10−4 and 10−3 s−1. Serrated plastic flow was observed from 250 to 350 °C at strain rate of 10−4 s−1, signifying occurrence of dynamic strain ageing (DSA). Characteristic features of DSA such as plateau/peak in yield and tensile strength, minima in ductility, negative strain rate sensitivity, and peak in internal friction were also observed. Activation energy for DSA was found equivalent to that for diffusion of nitrogen, hence nitrogen was considered to cause DSA. Dislocation substructure in the region of DSA revealed dislocation debris, kinks, bowing of dislocations and high density of dislocations. Irrespective of temperature from 200 to 450 °C there was formation of dislocation cell structure; however, cell size in the region of DSA was smallest. Fractographic analysis showed rosette type fracture at RT resulting from longitudinal splitting. It was associated with decohesion of interface of carbide particles and prior austenite grain boundaries. Non uniform and shallow dimples were observed in fibrous zones in the region of DSA. Contours of dual shear lip zone were observed at elevated temperatures from 200 to 450 °C.

Temperature and strain rate effect on tensile properties of 9Cr–1·8W–0·5Mo–VNb steel

Tensile tests have been carried out on 9Cr–1·8W–0·5Mo–VNb steel (grade 92) over wide ranges of temperature (300–923 K) and strain rate (3×10−3–3×10−5 s−1). The tensile strength of the steel decreased slowly with temperature at relatively lower temperature range, whereas rapidly in the higher temperature range with a plateau in the intermediate temperature range. The decrease in strain rate decreased the tensile strength of the steel both at lower and higher temperature ranges. Elongation to fracture and reduction in area increased with increase in temperatures and decrease in strain rate at higher temperature regime with a plateau in the intermediate temperature regime. The ductile mode of tensile failure has been observed in the investigated temperatures and strain rates. The plateau in the variation of tensile strength with temperature, the negative strain rate sensitivity of tensile strength and minimum in ductility of the steel in the intermediate temperature range are considered as a consequence of dynamic strain ageing. The rapid decrease in tensile strengths and increase in ductility at high temperatures have been attributed to the dynamic recovery.

A Probabilistic Study on the Ductility of Reinforced Concrete Sections

Although the current design codes apply reliability-based calibration procedures to evaluate safety factors for the strength based limit state, the safety factors used to ensure minimum ductility capacities are rather simple and are not resulted from a probability-based procedure. This study examines level of safety delivered by the current design codes with regards to providing minimum curvature ductility for reinforced concrete (RC) beams made with normal strength concrete. Reliability analysis results show that with regard to the strength limit state, the considered design codes are in good agreement with one another. However, there is considerable disparity in the level of safety provided for minimum curvature ductility amongst the codes. The provided reliability for the design to remain ductile is too low in some and just about acceptable in the others. This signifies the importance and the need to introduce reliability based methods of design for ductility.

Experimental study on the correlation between intermediate temperature embrittlement and equi-cohesive temperature

Intermediate temperature embrittlement is a general phenomenon in Ni-based superalloys and is frequently attributed to the equi-cohesive temperature. In this paper, the correlation between intermediate temperature embrittlement and equi-cohesive temperature is studied by the tensile ductility of high-purity Ni and binary Ni(Bi) alloy. Elevated-temperature tensile ductility of the Ni(Bi) alloy shows evident intermediate temperature embrittlement with minimum ductility between 700°C and 750°C, while that of high purity Ni does not. Room-temperature tensile ductility of Ni(Bi) alloy reveals clear minimum after a critical time of isothermal pre-annealing at 650°C or 750°C. The fracture morphologies of tensile samples are well consistent with the changes of their corresponding tensile ductility. Based on these results and reported results of Ni-based superalloys, it is clear that equi-cohesive temperature is insufficient to be a general interpretation for intermediate temperature embrittlement.

Investigation on relationship between intermediate temperature embrittlement and intergranular precipitation in AlCoCrCuFeNi alloy

The tensile ductility of AlCoCrCuFeNi alloy was studied in the temperature range of 373–1073 K. It could be shown that the system is composed of body centred cubic phase, and the alloy indicates evident intermediate temperature embrittlement with a minimum ductility between 573 and 873 K. The analysis carried out by the scanning electron and transmission electron microscopies revealed that there are precipitates within grains and at grain boundaries. Finally, the interpretation of precipitation in grain boundary for intermediate temperature embrittlement was then discussed.

Influence of Strain Rate and Temperature on Tensile Deformation and Fracture Behavior of Type 316L(N) Austenitic Stainless Steel

Tensile tests were performed at strain rates ranging from 3.16 × 10−5 to 3.16 × 10−3 s−1 over the temperatures ranging from 300 K to 1123 K (27 °C to 850 °C) to examine the effects of temperature and strain rate on tensile deformation and fracture behavior of nitrogen-alloyed low carbon grade type 316L(N) austenitic stainless steel. The variations of flow stress/strength values, work hardening rate, and tensile ductility with respect to temperature exhibited distinct three temperature regimes. The steel exhibited distinct low- and high-temperature serrated flow regimes and anomalous variations in terms of plateaus/peaks in flow stress/strength values and work hardening rate, negative strain rate sensitivity, and ductility minima at intermediate temperatures. The fracture mode remained transgranular. At high temperatures, the dominance of dynamic recovery is reflected in the rapid decrease in flow stress/strength values, work hardening rate, and increase in ductility with the increasing temperature and the decreasing strain rate.

Effect of non-equilibrium grain-boundary segregation of phosphorus on the intermediate-temperature ductility minimum of an Fe–17Cr alloy

The ductility of an Fe–17Cr alloy was examined experimentally in the range of 350–700°C. The results showed that the ductility of the alloy, evaluated by the reduction in area, had a minimum at an intermediate temperature of approximately 450°C. Auger electron spectroscopy demonstrated that the ductility minimum corresponded to a maximum in grain boundary segregation of phosphorus, which could be elucidated by the theory of non-equilibrium grain-boundary segregation. Thus, the intermediate temperature ductility minimum for the Fe–17Cr alloy could be explained on the basis of non-equilibrium grain-boundary segregation of phosphorus.

Seismic Evaluation of the ACI Code Provisions for Lap Splicing of Longitudinal Bars in R/C Rectangular Bridge Columns

In this study, the adequacy of the ACI code provisions for detailing of lap-spliced reinforcement in R/C rectangular bridge columns was numerically investigated. Pushover analysis was conducted on a total of 324 R/C rectangular bridge columns with lap-spliced reinforcement, using a modified version of previously developed computer program. The computer modeling is based on moment–curvature analysis of the column section with the inclusion of a bond/slip mechanism and concrete confinement model. In this study, the program was revised to integrate the effectiveness of lateral hoop reinforcement on both concrete confinement and lap-splice clamping. Constant axial load was applied in the analysis. The parameters involved in the study were lap-splice length, volumetric ratio of column hoops, and axial load ratio. The results indicate that the top lateral load–displacement characteristics of the column are enhanced when the lap-splice length at the column base increased. To ensure a minimum displacement ductility of 4.0 for the column, lap splices as short as 30φb (φb = longitudinal bar-diameter) should be avoided at expected plastic hinge locations. The best performance for a wide range of axial load ratios was exhibited by columns having the ACI’s classes A and B tension splices and laterally reinforced with hoops required by the ACI code for seismic design.

Tensile Properties and Deformation Characteristics of a Ni-Fe-Base Superalloy for Steam Boiler Applications

Ni-Fe-base superalloys due to their good manufacturability and low cost are the proper candidates for boiler materials in advanced power plants. The major concerns with Ni-Fe-base superalloys are the insufficient mechanical properties at elevated temperatures. In this paper, tensile properties, deformation, and fracture characteristics of a Ni-Fe-base superalloy primarily strengthened by γ′ precipitates have been investigated from room temperature to 1073 K (800 °C). The results showed a gradual decrease in the strength up to about 973 K (700 °C) followed by a rapid drop above this temperature and a ductility minimum at around 973 K (700 °C). The fracture surfaces were studied using scanning electron microscopy and the deformation mechanisms were determined by the observation of deformed microstructures using transmission electron microscopy. An attempt has been made to correlate the tensile properties and fracture characteristics at different temperatures with the observed deformation mechanisms.

Influence of Temperature and Strain Rate on Tensile Deformation and Fracture Behavior of P92 Ferritic Steel

Tensile tests were performed at strain rates ranging from 3.16 × 10−5 to 1.26 × 10−3 s−1 over a temperature range of 300 K to 923 K (27 °C to 650 °C) to examine the effects of temperature and strain rate on tensile deformation and fracture behavior of P92 ferritic steel. The variations of flow stress/strength values, work hardening rate, and tensile ductility with respect to temperature exhibited distinct three temperature regimes. The fracture mode remained transgranular. The steel exhibited serrated flow, an important manifestation of dynamic strain aging, along with anomalous variations in tensile properties in terms of peaks in flow stress/strength and work hardening rate, negative strain rate sensitivity, and ductility minima at intermediate temperatures. At high temperatures, the rapid decrease in flow stress/strength values and work hardening rate, and increase in ductility with increase in temperature and decrease in strain rate, indicated the dominance of dynamic recovery.

Characteristics of 30MnB5 boron steel at elevated temperatures

The hot ductility behavior of boron steel at high strain rates is of great importance because of its relationship with problematic brittleness observed during hot-forming processes. The proper hot-forming temperature must be established to manufacture high-quality products of complex geometries. In this investigation, the mechanical properties of 30MnB5 boron steel has been examined by hot tensile tests performed at temperatures ranging from 400 to 900°C, with 100°C increments, and at a strain rate of 0.083s−1. The deformation and fracture mechanisms under hot tensile testing were considered in relation to the testing data and structural observations performed by SEM on longitudinal specimen sections. The results indicate the minimum ductility of the tested boron steel observed as a function of deformation temperature.

Effect of Dynamic Strain Aging on the Mechanical Performance in Ti/STS439 Clad Materials

The effect of dynamic strain aging on the mechanical performance of Ti/STS439 clad material produced by hot rolling process was studied. A reaction layer with the thickness of approximately 3㎛ was formed during hot roll bonding process. The total elongation to fracture attains minimum values at about 400oC. The decrease of the ductility was accompanied by stress serrations in the stress-strain curves. At 400 oC, highly ductile and localized fracture with pin-point fracture surface occurred, suggesting the ductility minimum at 400 oC is extreme strain localization in the necked region due to low strain rate sensitivity

Non-equilibrium grain-boundary segregation of Bi in binary Ni(Bi) alloy

The minimum ductility of Ni(Bi) alloy caused by isothermal annealing at 750 °C or 650 °C is determined through tensile tests at room temperature. Tensile samples with minimum ductility display intergranular facets. Strong grain-boundary segregation of Bi in nanocrystalline Ni–Bi layer is observed by atom probe tomography. The minimum ductility, intergranular facets and grain-boundary segregation of Bi confirm the non-equilibrium grain-boundary segregation of Bi in Ni(Bi) alloy for the first time.